The fabrication of a composite structure may include the cutting of composite plies into predetermined shapes. Prior to cutting into shapes, the composite plies may be pre-impregnated with a mixture of resin and hardener (e.g., prepreg plies) and may be uncured or partially cured. The cut plies may be stacked on top of one another over a tooling surface (e.g., a mold surface) to form a composite layup. Each ply may be arranged such that fibers are oriented in a specific direction according to a ply stacking sequence. A vacuum bag may be placed over the composite layup and vacuum pressure may be applied to consolidate the composite layup to remove air and wrinkles from the composite layup and conform the composite layup to the tooling surface. The temperature of the composite layup may be elevated to reduce the viscosity of the resin so that the resin in each prepreg ply may flow and/or intermingle with the resin of adjacent prepreg plies.
The elevated temperature of the composite layup may initiate a curing process for the resin. During the curing process, the composite layup may be held at one or more elevated temperatures for a predetermined period of time to cure the resin into a solidified or hardened state. The curing of the resin may involve a cross-linking reaction during which the resin changes from a liquid to a solid. Heat may be generated due to the cross-linking reaction.
Up until the start of the layup process, thermoset prepregs must typically be stored at relatively cold temperatures (e.g., at 0 degrees F. or lower) to avoid the degradation of the resin and the additional curing of the resin that may otherwise occur if the prepreg were stored at room temperature. In this regard, a thermoset prepreg has a limited out-time which may be described as the amount of time that the prepreg may remain at room temperature before the prepreg starts to lose its manufacturability. For example, exceeding the out-time of a prepreg may result in a reduction of the tack or stickiness of the prepreg ply which is necessary to stabilize the ply against movement during layup. In addition, exceeding the out-time of a prepreg may result in a higher resin viscosity during the curing process, and which may compromise the ability to fully consolidate the composite layup to achieve the desired fiber volume fraction in the final composite structure.
Composite layups that are large and/or have complex geometry may require extended out-times to allow for the multiple processing steps that must be performed prior to final cure of a composite layup. For example, the fabrication of a large composite part may require several weeks of out-time at room temperature for cutting and laying up multiple prepreg composite plies, followed by vacuum bagging, consolidating, and other processing steps that may be required prior to final cure.
In conventional thermosetting resins, there is a trade-off between the length of out-time and the final required cure temperature and cure time. A thermosetting resin formulated to have a long out-time may require a relatively high cure temperature or a long cure time. A high cure temperature may present the risk of reduced properties of the cured resin and/or distortion in the shape of the final composite structure. A thermosetting resin may be formulated to have a relatively low cure temperature to avoid the drawbacks associated with high cure temperatures. However, a resin with a low cure temperature may have a relatively short out-time which may present challenges in fabricating composite parts that are large and/or have complex geometry.
As can be seen, there exists a need in the art for a resin system and method that allows for a relatively long out-time and which also has a relatively low cure temperature and/or relatively short cure time.